Although this would be a great feature, I can imagine how tricky it would be implement this feature.

I don't know how Avocode currently works, but I am guessing the sketch extension would take a 'snapshot' of each board and save it as a compressed image (jpg, .png, etc) and then overlay the image with boundaries that contain pieces of data (font-size, color, etc).

Personally, I couldn't find any 'snapshots' of boards. This means opening a sketch file directly would require parsing all the data contained in the .sketch file and then redrawing all the visuals.